Three common pitfalls of pressure calibrations

September 182014

By Roy Tomalino, Beamex

You are in Building 27 on the 3rd floor. It is time to calibrate a pressure transmitter. You have just finished the highest pressure point of the test. Now, it’s time to bring it back down. The easiest way to reduce the pressure is to vent the pressure pump. So, when you vent the pump, there is process fluid everywhere. That is not what you expected. How did this happen?

Calibrating pressure devices has its own unique challenges. One such challenge is knowing where to vent after you have pressured up your test line. In this scenario, a three-legged hose is used with one end going to the Device Under Test (DUT), one end to the pressure source (a hand pump, in this case) and one end to the pressure port on the calibrator. Venting a line under pressure creates a low pressure area similar to a vacuum effect on the contents of the line. It could be atmospheric air, nitrogen from a bottle or a process fluid (if present in the line, capillary or bourdon tube). If the process fluid is corrosive or dangerous, this can be an especially bad situation. Now you have to clean out the pump and probably the pressure transducer. However, you can avoid this in the future by venting at the DUT.

Another common challenge is finding zero. Isn’t zero always a mathematical zero? No. Not when you are dealing with gauge pressure. Absolute pressure has a zero, which would be a perfect vacuum found in space. When you zero a gauge pressure instrument, you zero it to the atmospheric pressure at that moment. Because this barometric pressure changes, your zero changes. This should be accounted for as a slight deviation due to random error in the process tolerance.

In looking further at pressure testing, the orientation of your DUT plays an important role during calibration and installation. A common design of pressure transmitters is to use a protective diaphragm seal with silicone fluid on the other side to register the pressure applied to the actual instrument sensor. Gravity will have an effect on the measurement with this arrangement. To demonstrate this effect, calibrate a pressure transmitter lying flat in a horizontal position. Then, simply change the position of the DUT to upright or vertical and run the test points again. You will notice a dramatic effect.

About the author
Roy Tomalino, Beamex Calibration Engineer, brings over 15 years of technical training experience. He has worked for Hewlett-Packard as a corporate trainer, leading new product introduction and managing worldwide training activities. He is an accomplished public speaker and facilitator. Roy holds a B.S. in Computer Information Systems from Regis University, Denver, CO; and A.A.S. Electronics Technology from Denver Institute of Technology, Denver, CO. Roy is also Six Sigma Green Belt certified. Today, Roy conducts educational training sessions and provides technical support to customers.